Long Term Evolution (LTE) is an improved universal mobile telecommunication system (UMTS) that provides higher data rate, lower latency and improved system capacity. In LTE systems, an evolved universal terrestrial radio access network includes a plurality of base stations, referred as evolved Node-Bs (eNBs), communicating with a plurality of mobile stations, referred as user equipment (UE). A UE may communicate with a base station or an eNB via the downlink and uplink. The downlink (DL) refers to the communication from the base station to the UE. The uplink (UL) refers to the communication from the UE to the base station. LTE is commonly marketed as 4G LTE, and the LTE standard is developed by 3GPP.
3GPP started a new study item, “Network Assisted Interference Cancellation and Suppression” (NAICS), to investigate the benefit on system throughput by leveraging receiver's capability of interference cancelation. Various types of interference cancellation (IC) receivers are shown to provide significant gain if some characteristics of interference are available at victim nodes. Commonly investigated IC techniques in literature may include symbol-level based IC (SL-IC) and codeword-level IC (CW-IC). SL-IC is an IC technique that detects interfering signal, which is supposed to be finite-constellation modulated, in a per-symbol basis. CW-IC is referred to that a receiver decodes and re-encodes interference codeword to reconstruct the contribution of the interference signal on its received signal. Comparing to SL-IC, a receiver needs more information on interference to access CW-IC, such as modulation and coding scheme (MCS) index and the rule scrambling the bit stream of interference.
Obtaining the interference characteristics, such as the modulation order or encoding rules of the interfering signal, is important for interference cancellation techniques. The characteristics could be either blindly detected by victim receiver or informed from network side. A number of 3GPP contributions under the study items of NAICS (e.g., 3GPP TR 36.866, 3GPP RP-130594, 3GPP RP-131241) showed the performance results of the advance receivers, including SL-IC, reduced-maximum likelihood (R-ML), and CW-IC receivers, with the assistance of network signaling.
The NAICS study item includes the following two main scenarios: 1) Intra-cell interference resulted from current single user (SU)-/multiuser (MU)-multiple input multiple output (MIMO) operation, and 2) Inter-cell interference based on deployment scenarios prioritized in LTE Rel-11, taking into account scenarios under Rel-12 work items/study items such as small cells. The study item has mostly focused on the inter-cell (or inter-point) case. It was concluded that SL-IC/R-ML receivers, with the network assistance of higher layer signaling of interference parameters (including any subset restriction) related to interference cells' common reference signal (CRS)/physical downlink control channel (PDCCH) transmission, could achieve noticeable gain over Rel-11 linear minimum mean square error (LMMSE)-interference rejection combining (IRC) receivers. The so-called NAICS receivers are also required to blind detect many parameters, including all the dynamic parameters to avoid any scheduling constraints that semi-static signaling may incur.
MU-MIMO has been defined in LTE since Rel-8 and Rel-9/10, but still no massive deployment yet. With the MU design in the current LTE, it is completely the BS's responsibility to minimize any MU interference after precoding which could be still based on limited UE codebook based feedback in frequency division duplexing (FDD). The residual MU interference is one of the reasons for limited system throughput gain for MU-MIMO over SU-MIMO where inter-layer interference can be more effectively cancelled with the complete knowledge for both layers. It is expected that the Rel-12 NAICS receivers can at least improve the MU performance if MU interference information can be provided or accurately detected. MU operation is expected to become more suitable with the increasing interest in the deployment of 4-TX and 8-TX BSs. MU-MIMO has also received a lot of interest in IEEE 802.11 to better serve multiple UEs in a conference room scenario with good fairness.
The concept of a joint optimization of MU operation from both transmitter and receiver's perspective has the potential to further improve MU system capacity even if the transmission/precoding is non-orthogonal. For example, the simultaneous transmission of a large number of non-orthogonal beams/layers with the possibility of more than one layer of data transmission in a beam. Such joint TX/RX optimization associated with adaptive TX power allocation and CW-IC receiver is referred to as non-orthogonal multiple access (NOMA). Joint TX/RX optimization might require standardization effort on the signaling and feedback aspects. Hence, it is important to study the tradeoff in terms of system performance, complexity, and singling overhead.
In a radio communication system, a receiver of a UE can be very complex and unreliable if it must detect or estimate all the characteristics of an interfering signal, especially when the parameters related to the interfering signal can be very dynamic. A signaling method is sought for the network to assist the victim receiver by providing information related to the interfering signal. By doing so, the computation complexity of the victim receiver in blindly detecting the interfering signal characteristics can be reduced, and the reliability of the signal detection can be improved.